The Far-Target Locator measures target Azimuth relative to Spin North and target elevation from the soldier position. The state-of-the art is a gyrocompass that is large, heavy, costly and requires a stable, aligned platform. What is required for soldier utility is a small, low-weight, portable unit that uses low power. The portability option is a magnetic compass, however it is sensitive to metals and aligns to Magnetic North, which is not stable, and therefore requires frequent corrections.
The traditional gyrocompass procedure applied to MEMS (micro-electromechanical systems) is to align the input axis of a first gyro with Spin North by maximizing the amplitude response of the gyro. The same gyro is then rotated to align with its cross-axis (90 degrees rotation) at which angle no rate exists to find its minimum response. In order to obtain the precision required, a smallest amplitude change corresponding to the angle requirement needs to be detectable. Once aligned with Spin North, the platform is then slewed to align with the target. The angle between Spin North and target is the Azimuth to the target. The Azimuth angle, however, is determined by integrating the rotation rate of a second (slew) gyro with Input Axis along the Vertical. The accuracy of the measurement is due to the accuracy with which Spin North is determined, which depends on the bias stability of the MEMS gyros. We refer to the above description of an Amplitude Gyrocompass to contrast it with the Phase Gyrocompass of this invention. The Azimuth accuracy also depends on the bias stability of the slew gyro and its white noise, which leads to angle random walk error.
Note that bias instability includes short-term (1/f) Bias Instability, Drift Instability and Bias Offset.